1. Field of the Invention
The present invention relates to a noise-reduction method and apparatus, and, more particularly, to a noise-reduction method and apparatus in which it is possible to prevent the occurrence of edge artifacts by applying different filter weights according to the activity levels of regions in an image, and in which it is also possible to prevent the occurrence of an outlier phenomenon by applying different filter weights according to outlier probabilities.
2. Description of the Related Art
A high-sensitivity photographing function is generally required to perform photographing in a low-illumination environment and during night time. In recent years, the demand for the high-sensitivity photographing function has steadily grown.
One of the major drawbacks of the high-sensitivity photographing function is that noise is amplified when using the high-sensitivity photographing function. Much research has been conducted on ways to reduce noise.
Noise-reduction methods using the discrete cosine transform (DCT) or wavelet transform can achieve high noise-reduction performance. However, such DCT- or wavelet transform-based noise-reduction methods are highly likely to generate ringing artifacts along edges in an image when being used to transform the frequency properties of the image. In addition, it is very complicated to apply such DCT- or wavelet transform-based noise-reduction methods to typical cameras.
Noise-reduction methods such as a total variation optimization method using iteration and an anisotropic diffusion method can also achieve high noise-reduction performance, but it is difficult to apply them to typical cameras. Moreover, the total optimization method and the anisotropic diffusion method require a considerable amount of hardware resources.
Spatial filters, however, generally have a low level of complexity and require less hardware resources. Thus, spatial filers have been widely used in cameras for reducing noise. Examples of spatial filters include Wiener/minimum mean square error (MMSE) filters (“Method of Filtering Image Noise using Pattern Information,” US 2007/0133895 A1 and “Spatio-Temporal Joint Filter for Noise Reduction,” U.S. Pat. No. 6,999,634 B2), sigma filters (J. S. Lee, Digital Image Smoothing and the Sigma Filter, 1983), and bilateral filters (C. Tomasi, R. Manduchi, Bilateral Filtering for Gray and Color Images, 1998, Method for Bilateral Filtering of Digital Images US 2005.0025378). However, spatial filters may produce edge artifacts and outliers (also referred to as impulse noise), which appear in an image as dots, when being used to reduce noise caused by high-sensitivity photography.
FIG. 1 illustrates edge artifacts caused by the use of a conventional Wiener filter, and FIG. 2 illustrates outliers caused by the use of a conventional Sigma filter.
Referring to FIG. 1, edges in an image obtained using a Wiener filter appear unclear and are distorted. Referring to FIG. 2, an image obtained using a Sigma filter includes many outliers which appear as dots.